This paper presents the mechanism of crack growth from a 3-D surface flaw in granite specimens subjected to both uniaxial and biaxial compressions through the use of strain measurement and the acoustic emission (AE) technique. Four types of crack patterns were observed under both compressions: wing cracks, petal cracks (crack growing along the edge of a flaw plan), compressive cracks and anti-wing cracks (crack growing from the same tip of the wing crack but at the opposite side). The anti-wing cracks initiated after the wing cracks initiated, but the growth rate is 4.5 to 10 times faster than that of the wing cracks under both compressions. The strain records indicated that when the anti-wing cracks propagated, the strain values of the wing crack at the other tip of the same flaw reduced. The average released AE energy (energy/event) from the growth of the wing crack is the smallest (600aJ to 6,478aJ), but the average released energy from the growth of the compressive cracks is the highest (10×103aJ to 207×103aJ). The crack growth mechanism of a 3-D surface flaw is strongly affected by the ratio of the flaw depth ‘d’ to specimen thickness ‘t’. The growth of anti-wing cracks is dominant in all the specimens containing a 3-D surface flaw. Thus, the analysis of antiwing cracks cannot be ignored in the study of a sliding 3-D surface flaw model.
Numerous fractures exist in the earth crust. The size of fractures is varied from several millimeters to several kilometers. Some of the fractures which expose on the ground surface are defined as the surface fractures, while some of them which embed in the ground are defined as the internal fractures. Under compression stresses, cracks propagated from the pre-existing fractures may result in rock failure or earthquakes.